1. Field of the Invention
The present invention relates to a coating composition which is a spray powder for the production of wear resistant, burn trace resistant and break-away resistant coatings for the bearing faces of machine parts that are to be subjected to friction stresses and wear, and in particular, to a coating composition for friction-reducing coatings for wearing surfaces of machine parts, for example, piston rings, the fire lands of pistons of high rpm and highly stressed internal combustion engines, and the bearing faces of synchronizing rings.
2. Background of the Art
In order to increase wear resistance, the bearing faces of machine parts exposed to sliding friction have been provided with hard coatings, such as electrochemically deposited, chromium metal coatings or thermally-applied sprayed coatings of molybdenum metal. The molybdenum metal coating may be applied to the wearing surface of a machine part either over the entire area as an overcoating or only partially, into grooves in unilaterally or bilaterally chambered form. The sprayed coatings may be applied by the known flame spraying technique using a plasma-arc gun.
Sprayed-on molybdenum coatings on, for example, the bearing faces of piston rings have excellent burn trace resistance and, due to their good oil retention properties, are superior to electrochemically deposited chromium coatings layers. Molybdenum coatings, however, are less wear resistant than electrochemically deposited chromium layers and, primarily due to increased porosity and brittleness during engine operation, larger regions of the coating may delaminate and break-away, and cause engine malfunctions.
In order to further increase wear resistance, molybdenum has been alloyed with other elements and hard substances, such as the nitrides, oxides or carbides of metals, have been added to the powdered alloy. For example, according to German Patent Number (DE-AS) No. 2,433,814, molybdenum is alloyed with from 0.5 to 45 weight percent of a heavy metal, i.e., a metal of atomic weight greater than sodium, and from 0.8 to 10 weight percent silicon. According to German Laid-Open Patent Application No. (DE-OS) 2,841,552, molybdenum containing from 3 to 40 weight percent of a molybdenum alloy is disclosed. According to U.S. Patent No. 3,556,747, the disclosed spray powder is composed of a mixture of from 5 to 50 weight % molybdenum, from 37.5 to 95 weight % chromium carbide Cr.sub.3 C.sub.2 and up to 23.75 weight % of a nickel-chromium alloy.
Due to the addition of hard substances, the molybdenum alloy layers produced with spray powders according to these references are significantly more wear resistant. The high percentage of hard substances, however, has the disadvantageous result of increasing the wear of the counter-running faces which frictionally mate with wearing surfaces coated with these friction-reducing coating compositions. Particles abraded from the counter-bearing surfaces or particles of hard substances from the coated surface itself may cause further damage to the machine.
Responsive to the foregoing problem, a piston ring coating was developed, according to U.S. Patent No. 4,334,927, which was prepared for a coating composition composed of 51 weight % molybdenum, from 5 to 20 weight % chromium carbide Cr.sub.3 C.sub.2 and from 32 to 51 weight % of a nickel-chromium alloy. Due to the low percentage of the hard carbide substance, wear of counter-bearing faces is reduced and, at most, only particles which cause a low level of abrasive wear result, however, the wear resistance of the sprayed-on coatings themselves is reduced. Such lowered wear resistance is then no longer adequate for coated parts subjected to high frictional stresses, for example, in high rpm internal combustion engines.
According to German Laid-Open Patent Application No. (DE-OS) 3,247,054, molybdenum carbide Mo.sub.2 C was used as the hard substance and spray powders for producing such coatings are composed of from 20 to 60 weight % molybdenum, from 25 to 50 weight % molybdenum carbide and up to 30 weight % of a low melting point nickel-chromium alloy. Good wear behavior was noted for these coatings and also with respect to their counter-running partners. During engine operation, particularly in internal-combustion engines, coated piston rings exhibited satisfactory strength values. However, for engines subject to extreme frictional stresses, coated piston rings having even further improved strength, wear resistance and break-away resistance would be advantageous.